Electric rail systems which are supplied with electricity from overhead power lines rely on a pantograph attached to an electric locomotive to provide a current pathway between an overhead power line and one or more electric motors. The pantograph supports a carbon brush that extends perpendicular to the direction of the overhead line. In order to maximise the life of the carbon brush and avoid damage, the overhead lines do not extend exactly parallel with the tracks on which the locomotive run, but rather are supported to meander within predetermined boundaries between the tracks. In this way the overhead line does not contact the carbon brush in the same location but rather the contact point moves from side to side along the brush as the locomotive traverses along the rail.
During the construction of the electric rail system, the location of the overhead lines is predetermined and recorded. From time to time when conducting maintenance of an electric rail system it is important to make measurements to ensure that the power lines are in their predetermined location.